1. Field of the Invention
The invention relates to multi-layer thin film structures and fabrication techniques for forming electrical devices, and in particular, for forming electrochromic devices for modulating visible, and, more significantly, tuning and modulating infrared radiation.
2. Background Information
An electrochromic device (‘ECD’) is used to modulate light and commonly comprises two active layers separated by an electrolyte (sometimes referred to as an ‘EL’ or ‘EL layer’) through which a selected mobile ion can be controllably moved from one of the active layers to the other. Both of the active layers can contain varying concentrations of the mobile ion. In at least one of the active layers, commonly referred to as the ‘electrochromic layer’ (‘EC layer’), changes in the ion concentration are accompanied by changes in both the amount of light absorbed and the spectral variation of that absorption—i.e., changes in color and transparency. In the other active layer, commonly referred to as the ‘ion storage layer’ (IS layer), changing the ion concentration may or may not be accompanied by notable changes in transparency or coloration. If the IS does not show a visible change, it is referred to as a ‘passive ion storage layer.’ If the applied voltage causes an optical change in the IS layer similar to the one occurring in the EC layer, it is called a ‘complementary ion storage layer.’
Operation of an electrochromic device generally involves applying a voltage between the electrochromic and ion storage layers so as to cause the mobile ions to move from one layer to the other and to thereby selectively alter the coloration and transparency of the EC layer. This is commonly described in terms of the EC layer, or of the entire ECD, being switched between ‘bleached’ and ‘colored’ states. Longstanding concerns of ECD development include finding film systems in which the transition between the two states is relatively speedy, can be accomplished with relatively low voltages, and provides a large change in transparency between the bleached and colored states. Another performance concern is optimizing the ‘self bleaching time’ of coloration decay after removal of the voltage.
In addition to the EC, EL, and IS layers, there are two electrode layers in an ECD, one abutting the EC layer and the other abutting the IS layer. The electronic conductivity of these layers is often an important consideration and can impose limitations on the switching speed and the uniformity of coloration.
Additional layers may be required in various ECD embodiments in order to protect the ECD during fabrication and service, or to prevent internal degradation. The most common additional external layer is an overcoat serving as an ambient moisture barrier. The overcoat layer is comprised of a transparent material, such as an oxide or a polymer material, which may be applied subsequent to vacuum processing by dipping, spraying, spinning, screening, or any other suitable approach.
Although the term ‘infrared’ generally encompasses electromagnetic radiation extending from the long wavelength limit of visible radiation (e.g., about 780 nm) to the low wavelength boundary of the microwave region (e.g., about 1 mm), the principle areas of interest to the present invention are the ‘near IR,’ ‘mid IR,’ and ‘long-wave IR’ regions that, taken together, extend from about 780 nm to about 50 microns. One reason for interest in this spectral region is that it encompasses substantially all the energy radiated by a black body at 300 degrees Kelvin, which is the optimal internal temperature of a space satellite. Because the external surfaces of a space satellite are exposed to the extreme temperatures of space, regulating the internal satellite temperature through surface emissivity control is an area of interest.